According to a method known from EP 05 80 267 A the drive motor of the yarn feeding and measuring device first is strongly decelerated to the low speed of the crawl phase and then is rotated further at slow speed for a predetermined rotation angle or a predetermined time duration and then is stopped at the end of the crawl phase to prevent the formation of loops in the yarn between the storage bobbin and the winding element. The strong deceleration of the drive motor and the inertia of the yarn result in a relaxation of the yarn between the storage bobbin and the winding element which relaxation may lead to a loop formation. This danger is particularly high when the drive motor is decelerated very strongly from high or maximum speed. During the subsequent restart of the drive motor depending on consumption the yarn is stretched abruptly which might cause a yarn breakage. The crawl phase directly continuing the deceleration phase either prevents that a loop will be formed or stretches an already formed loop.
According to the method known from EP 02 61 683 A a crawl phase directly continues a deceleration of the drive motor of the yarn feeding device down to crawl speed which crawl phase then is carried on for e.g. 200 ms. The purpose of the crawl phase is to prevent the occurrence of kinks in the yarn between the winding element and the storage surface, or to suppress slackness of the yarn in this area which may be caused by a backturn motion of the winding element counter to the normal winding direction.
Both known methods are based on the task either to suppress a loop formation occurring with the deceleration of the drive motor or to remove a loop already prior to the stop of the winding element. The crawl phase conventionally is controlled by a software pre-adaptation of the control device, however, by doing so it may be complicated to determine the start of the crawl phase precisely enough with the still running drive motor, since the drive motor may have differing run out phases depending on the operational conditions and the yarn quality, respectively. To assure that during the crawl phase a sufficient yarn length is pulled into the yarn feeding device, the crawl phase is adjusted for security reasons longer than necessary. For weaving machines operating with high insertion frequencies and extremely high yarn speeds in the yarn feeding device, however, it is important to stop the drive motor as rapidly as possible, in case that the number of windings stored in the yarn feeding device reaches the maximum and when at the same time no yarn consumption takes place. A crawl phase, which, however, is too long for safety reasons easily may result in an overfilling of the yarn feeding device. The static friction to start the yarn from stand still must be overcome in any case for a restart and also the static start friction in the drive motor.
It is an object of the invention to provide a method of the kind as disclosed herein which allows a correct yarn control at the inlet side of a weaving machine yarn feeding device in a simple and different manner.
The invention is considering the recognition that a relaxation of the incoming yarn during deceleration of the drive motor first neither is particularly critical for the yarn nor for the yarn feeding device or the weaving machine downstream of the yarn feeding device, but only is critical for the subsequent re-start. The yarn relaxation occurs due to the inertia or the elasticity of the yarn. Bearing this recognition in mind, the crawl phase for a predetermined rotation angle or a predetermined time duration thus is carried out after the stand still condition and so to speak in peace. For that reason, according to the method, first the drive motor is brought to stand still completely, and particularly because of the danger of an overfilling as rapidly as possible, and then the time duration available between the stand still condition and the subsequent re-start depending on consumption is utilised to carry out the crawl phase for the precise time duration or the exactly needed rotation angle, respectively. This is simpler in terms of control technique. Experience has namely proven that always a sufficiently long time duration will be available after the drive motor has been stopped from high speed. A yarn relaxation is tolerated intentionally, which might be caused by inertia, by a backturn motion of the winding element due to the yarn tension, or for other reasons, in order to first assure a rapid stop of the drive motor and to avoid overfilling of the yarn feeding device, and a measure is started first at a later point in time to omit the potential dangers of a formed loop which was particularly dangerous for the subsequent re-start.
According to the method the crawl phase is carried out for a predetermined time duration and with a predetermined speed timewise between a stand still depending on consumption and the subsequent re-start also depending on consumption or yarn demand. In this case the crawl phase speed either may be constant or variable.
Particularly, the weaving pattern during a multi-colour weaving operation may dictate longer stop pauses for a yarn feeding device feeding a certain colour. In the case that the yarn tends to relax during a longer pause, e.g. by pulling back the winding element counter to the normal winding direction, it may be expedient to associate the crawl phase timewise not to the braking phase but to the re-start, i.e., to carry out the crawl phase first immediately prior to the subsequent re-start such that a correct yarn control is guaranteed when the drive motor is restarted.
In this case it may be expedient to adjust the timewise termination of the crawl phase exactly to the point in time or even shortly after the point in time of the subsequent re-start. This may result in the advantage that the incoming yarn still may be in motion during the subsequent re-start and has not reached a condition in which the yarn or the winding element, respectively, has to overcome static starting friction. A sliding transition from the crawl phase with optionally increasing speed into the subsequent re-start is particularly advantageous for delicate yarn qualities. In this case no static starting friction has to be overcome in the drive motor as well such that the drive motor may accelerate more forcefully.
The re-start phase of the drive motor basically may contain the previous crawl phase in order to fully accelerate without stand still already from the crawl phase speed. Such a combined re-start phase e.g. is triggered by the consumption depending start signal for the drive motor and is then made by a corresponding control routine. In this case the crawl phase does not need to be controlled separately. There is no static starting friction which has to be overcome. The drive motor can be accelerated more efficiently.
Basically it is expedient to know the point in time of the subsequent and consumption depending re-start in order to precisely adapt the crawl phase thereto. This is achieved according to a further variant of the method by providing weaving pattern dependent information and to transmit the same to the control device, the information indicating at which point in time or at which rotation angle value, e.g. of the driving,shaft of the weaving machine or after how many upcoming insertion cycles the driving motor of this yarn feeding device again has to re-start. On the basis of this information the crawl phase can be carried out precisely and optimally, particularly also such that a sliding transition will take place from the crawl phase into the consumption depending subsequent re-start.
By this pre-information of the control device of the yarn feeding device a prerequisite is set for adjusting the timewise termination of the crawl phase even shortly before, precisely on or shortly after the point in time at which the consumption depending subsequent re-start will take place. This allows not only to effect a sliding transition into the subsequent re-start, but even allows to adjust the crawl phase precisely such that then only so much yarn is pulled into the yarn feeding device sufficient to compensate for a potential loop formation.